CFA633-RDI-KS datasheet(32/88 Pages) CRYSTAIFONTZ

32 / 88 page

Crystalfontz America, Inc.

Data Sheet Release Date 2012/02/21

www.crystalfontz.com

CFA633-RDI-KS LCD Module (Hardware v2.0 / Firmware s2v1)

February 2012

Page 32

Since the on-time is dynamically stretched by the CFA633-RDI-KS to force the fan to produce two tachometer edges,

the result is that the fan will resist stalling as power is reduced towards 0% and the RPM is being measured.

Here is a graph of fan RPM vs. the fan power setting for a typical high-performance 80 mm fan (for example, Delta

FFB0812SHE):

Figure 14. Graph showing fan PWM power (% on time) for various fan RPM

Typically if the fan speed is not at 100% then it is being controlled by the host software to drive a temperature sensor to

a given reading in a closed-loop arrangement. In this case, the temperature, rather than the fan speed would be

monitored for out-of-range conditions. If the temperature is within specification, you really do not care how fast the fan

is turning. In an unattended system, it may be a good idea to set each fan to 100% for a few seconds during a test cycle

— perhaps once a day or once a week — and log the steady-state RPM attained by the fan. If that steady state RPM

were higher (this can be caused by a blocked airflow) or lower than expected (perhaps the fan's bearings are failing), a

maintenance warning could be generated by the host software.

We tested the CFA633-RDI-KS with a large range of fans and had good results. However, you are responsible for

determining if the control and monitoring methods employed by the LCD module are acceptable for your application. In

particular, if a fan's power is set too low, it may stall or fail to start, providing no cooling. Using a PWM to control fan speed

is generally accepted; however we make no claims that it is compatible with any particular fan or that it does not affect

the lifetime of the fans. Some higher torque fans (especially the ball-bearing models) may click, buzz, or growl at low

power settings due to the torque in the fan going from positive to negative in each PWM cycle. If you limit the power

NOTE

For safety, enable the fan power fail-safe (see command 25 (0x19): Set Fan Power Fail-

Safe (Pg. 51)) on any fans involved in host-based speed control. By enabling the fail-safe on a fan

that is being used in closed-loop control through host software, the CFA633-RDI-KS will turn that

fan to 100% if the host fails to update the power of the fans within a given time interval. For

instance, if the communications cable is dislodged, the host operating system hangs, or cooling

control process is terminated, the CFA633-RDI-KS will automatically force those fans to 100%, pre-

venting potential equipment damage due to lack of cooling.

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